Fine metal particles are recognized to be potentially superior magnetizable pigments for magnetic recording media. One obstacle to fully realizing that potential is the high reactivity of the particles caused by their fine size (they are typically less than 1000-1500 angstroms in diameter). This reactivity makes the particles susceptible to oxidation or other deterioration, even when dispersed in binder material in a magnetic recording medium. The result is that the recording medium may not be environmentally stable; that is, it may lose a substantial percentage of its magnetic properties when stored and used in normal ambient environments.
Several ideas for providing environmental stability have been proposed, but insofar as known none of them gives evidence of real success: either these proposals do not attain a desired level of environmental stability, or they unduly reduce other properties of magnetic recording media incorporating the particles, as by reducing the magnetic properties of the particles. An example of the latter is Little et al, U.S. Pat. 3,535,104, which suggests improving environmental stability by alloying chromium into the particles. The price for that environmental stability is a significant reduction of saturation magnetic moment and other magnetic properties of the alloyed particles (more than 30 percent loss in magnetic moment by adding 20 weight-percent chromium). With such a reduction in the initial or base recording properties of the particles, it makes little difference if those initial properties are substantially retained after environmental exposure; whether or not the properties are retained, the full potential of the fine metal particles is not realized.